Fire hydrants are used in virtually every community in the country to allow fire fighting apparatus to obtain a water supply for fighting fires. Unfortunately, traditional hydrant designs were all too easy to operate by unauthorized users such as children wishing to play in the water, illicit water-suppliers, and simple vandals wasting water for no particular reason.
As many areas become increasingly sensitive to the loss of water from the municipal system, various protection methods evolved to make it harder for unauthorized users to access hydrants.
One design which has proven important commercially is the cap patented in Graham, U.S. Pat. No. 3,929,152, assigned to Hydra-Shield Manufacturing, Inc. This design used a domed cap with a plurality of grooves which required a special wrench having a gripping portion with shoulders for engaging the grooves. As this design became widely used, however, vandals learned that the cap could be removed using a strap wrench, defeating the security.
Over a period of years the Graham design was modified to include a slip ring concentrically located around the cap. If a vandal attempted to remove the cap with a strap wrench, he could only rotate the slip ring around the cap, without loosening the cap. As the modified cap became common over time, vandals learned that it, too, could be defeated by simply hitting the slip ring with a hammer until it was bent and would no longer rotate, at which point a strap wrench would once again suffice to remove the cap.
Although this modification to Graham was not patented, to the best of the present inventor's knowledge, Franceschi, U.S. Pat. No. 3,935,877 shows the use of a basically identical slip ring arrangement (called a "Saturn Ring" in that patent) secured to a cap by a snap ring to prevent turning of a hydrant valve. The Franceschi patent also shows an alternative to the Graham grooves, in that the cap has a plurality of gripping surfaces around the domed cap, which are adapted to mate with the matching specialized wrench.
In order to prevent the deformation of the slip ring design, which problem had become obvious over the years of use, Hydra-Shield modified the design to use a forged carbon steel material for the cap and the slip ring, heat treated to a Rockwell "C" hardness at least in the range of 50-62 (the specification calls for a preferred range of 58-62, to a depth of 0.03"). This was patented as Stehling, U.S. Pat. No. 5,033,501.
Stehling was an improvement over the modified Graham cap, in that the heat treated carbon steel hardened slip ring makes it more difficult to deform the slip ring and thus fuse it to the cap. However, the carbon steel required by the Stehling patent is prone to corrosion, which necessitated making provision for the injection of lubricant between the slip ring and the cap (see Stehling, column 4, line 61, to column 5, line 9). In practice, however, fire hydrant caps rarely if ever receive the maintenance attention needed to periodically inject lubricant into the caps. As years pass, the slip rings eventually corrode under the influence of the weather until they fuse to the cap, and the purpose of the slip ring is thus defeated.
Also, the heat treating of the carbon steel results in the specified hardness only on the surface of the slip ring (to a depth of 0.03" according to the specification of the patent).